1. Field of the Invention
The present invention relates to a displacement information detection apparatus. For example, the present invention is suitable for a Doppler velocimeter which radiates a laser beam onto a moving object or fluid (to be referred to as a "moving object" hereinafter), and detects a shift in frequency of scattered light which is Doppler-shifted according to a moving velocity of the moving object, thereby measuring the moving velocity of the moving object in a noncontact manner.
2. Related Background Art
As a conventional apparatus for measuring a moving velocity of a moving object with high precision in a noncontact manner, a laser Doppler velocimeter is used. The laser Doppler velocimeter is an apparatus for radiating a laser beam onto a moving object, and measuring the moving velocity of the moving object by utilizing an effect (Doppler effect) in that the frequency of scattered light from the moving object is shifted in proportion to the moving velocity of the moving object.
FIG. 1 is a diagram for explaining a conventional laser Doppler velocimeter.
In FIG. 1, a laser beam emitted from a laser 1 is converted into a collimated beam 3 by a collimator lens 2, and the collimated beam is split into a transmission beam 5a and a reflection beam 5b by a beam splitter 4. These beams 5a and 5b are respectively reflected by reflection mirrors 6a and 6b, and the reflected beams are then radiated from different directions at an incident angle .theta. on a moving object 7, which is moving at a speed V. Scattered lights from the moving object 7 are detected by a photosensor 9 via a focusing lens 8. In this case, the frequencies of scattered lights based on the two beams are Doppler-shifted by +.DELTA.f and -.DELTA.f in proportion to the moving speed V. If the wavelength of the laser beam is represented by .lambda., a frequency change .DELTA.f can be expressed by the following equation (1): EQU .DELTA.f=V.multidot.sin(.theta.)/.lambda. (1)
The scattered lights Doppler-shifted by +.DELTA.f and -.DELTA.f interfere with each other, and cause a change in brightness pattern on the light-receiving surface of the photosensor 9. The frequency F of the scattered light is given by the following equation (2): EQU F=2.multidot..DELTA.f=2.multidot.V.multidot.sin(.theta.)/.lambda.(2)
When the frequency F of the photosensor 9 (to be referred to as a "Doppler frequency" hereinafter) is measured from equation (2), the moving velocity of the moving object 7 can be obtained.
In the conventional laser Doppler velocimeter, the Doppler frequency F is inversely proportional to the wavelength .lambda. of the laser, as can be seen from equation (2). Therefore, the laser Doppler velocimeter must employ a laser light source having a stable wavelength. As a laser light source capable of performing continuous oscillation, and having a stable wavelength, a gas laser such as an He-Ne laser is often used. However, a laser oscillator itself is bulky, and requires a high voltage in a power supply. As a result, the apparatus tends to become bulky and expensive.
A laser diode (or a semiconductor laser) used in a compact disk device, a video disk device, an optical fiber communication, and the like is very small, and is easily driven. However, the laser diode has temperature dependency.
FIG. 2 is a graph for explaining a standard temperature dependency of a laser diode (quoted from '87 Mitsubishi Semiconductor Data Book; Optical Semiconductor Element Edition). A portion where the wavelength is continuously changed is mainly caused by a change in refractive index of an active layer of the laser diode according to a change in temperature, and changes at a rate of 0.05 to 0.06 nm/.degree.C. On the other hand, a portion where the wavelength is discontinuously changed is called longitudinal mode hopping, and changes at a rate of 0.2 to 0.3 nm/.degree.C.
In order stabilize the wavelength, a method of controlling the laser diode to have a predetermined temperature is normally employed. In this method, temperature control members, such as a heater, a radiator, a temperature sensor, and the like must be attached to the laser diode with a low heat resistance, and precise temperature control must be performed. As a result, the laser Doppler velocimeter becomes relatively large in size, and its cost is increased. In addition, instability in wavelength caused by the longitudinal mode hopping cannot be perfectly eliminated.
As a laser Doppler velocimeter which can solve the above-mentioned problems, the following system (to be referred to as a G-LDV hereinafter) has been proposed. In this system, a laser beam is incident on a diffraction grating, and of diffraction lights obtained from the diffraction grating, two, i.e., +nth-order and -nth-order (n=1, 2, . . . ) diffraction lights other than the 0th-order diffraction light are radiated on a moving object at the same crossing angle as an angle defined by the above-mentioned two beams. Scattered lights from the moving object are detected by a photosensor.
FIG. 3 shows diffraction lights obtained when a laser beam I is incident on a transmission type diffraction grating 10 having a grating pitch d, in a direction perpendicular to an arranging direction t of the gratings. In this case, a diffraction angle .theta..sub.0 is given by: EQU sin.theta..sub.0 =m.lambda./d
where m is the diffraction order (0, 1, 2, . . . ), and .lambda. is the wavelength of the laser beam.
Of these lights, .+-.nth-order lights other than the 0th-order lights are expressed by the following equation: EQU sin.theta..sub.0 =.+-.n.lambda./d (3)
FIG. 4 is a diagram for explaining a Doppler velocimeter, proposed by U.S. Ser. No. 501,499, for radiating the .+-.nth-order lights on the moving object 7 from different directions to have an incident angle .theta..sub.0. From equations (2) and (3), the Doppler frequency F of the photosensor 9 is given by: EQU F=2Vsin.theta..sub.0 /.lambda.=2nV/d (4)
More specifically, the frequency F is inversely proportional to the grating pitch d of the diffraction grating 10 independently of the laser beam I, and is proportional to the moving velocity of the moving object 7. Since the grating pitch d can be sufficiently stable, the Doppler frequency F is proportional to only the moving velocity of the moving object 7. Note that the same applies to a case wherein the diffraction grating 10 comprises a reflection type diffraction grating.